The present invention relates to a current switching circuit and, more particularly, to a current switching circuit having a wide output voltage range and equal output current values.
As a conventional current switching circuit, a current switching circuit, shown in FIG. 3, for switching and outputting a constant current to a first terminal O.sub.A or a second terminal O.sub.B is known.
In this case, a current which is proportional to a current flowing from a constant current source I is obtained as a collector current of a transistor Q.sub.9 by transistors Q.sub.1, Q.sub.4, and Q.sub.9 and resistors R.sub.1 and R.sub.2 constituting a current mirror circuit.
The collector current of the transistor Q.sub.9 is switched by transistors Q.sub.2 and Q.sub.3 differentially operated by a control signal Q.sub.A and an inverted signal Q.sub.B thereof to obtain a switching output from the output terminal O.sub.A or O.sub.B.
In the conventional current switching circuit, in order to operate the output terminal O.sub.A and O.sub.B as constant current terminals, even when the output voltages of the output terminals O.sub.A and O.sub.B are minimum, the operations of the transistors Q.sub.2, Q.sub.3, and Q.sub.9 must be set within a non-saturation region. For this reason, even when the voltages of the output terminals Q.sub.A and Q.sub.B are optimally selected, a voltage of about 1 V must be generated with reference to the ground point. That is, a voltage obtained by subtracting about 1 V from the power supply voltage V.sub.CC is a dynamic range. When the power supply voltage V.sub.CC is low, especially, the dynamic range is narrowed.
FIG. 4 shows another conventional current switching circuit described in Japanese Patent Laid-Open No. 3-160603 filed by the present applicant. A voltage generated by a resistor R.sub.1 and a base-emitter voltage of a transistor Q.sub.1 by means of a current from a constant current source I is applied to the base of a transistor Q.sub.2 through a transistor Q.sub.7 constituting the switching circuit, and the emitter of the transistor Q.sub.2 is grounded through a resistor R.sub.2, i.e., connected to the other terminal of the resistor R.sub.1. A transistor Q.sub.15 and an inverter INV1 are operated to turn off the transistor Q.sub.2, and reference symbol C.sub.1 denotes a control input terminal. Similarly, a voltage generated by the resistor R.sub.1 and the base-emitter voltage of the transistor Q.sub.1 is applied to the base of a transistor Q.sub.3 through a transistor Q.sub.8 constituting the switching circuit, and the emitter of the transistor Q.sub.3 is grounded through a resistor R.sub.12. A transistor Q.sub.16 and an inverter INV2 are operated to turn off the transistor Q.sub.3, and reference symbol C.sub.2 denotes a control input terminal. A transistor Q.sub.4 is a compensation circuit for a base current. A transistor Q.sub.10 is used for compensating for voltage drop caused by the ON resistance of the transistor Q.sub.7 or Q.sub.8 and the base current thereof, and the transistor Q.sub.10 is always set in an ON state.
However, when the current value of the constant current source I is represented by I, and the resistances of the resistors R.sub.1, R.sub.2, and R.sub.12 are represented by R.sub.1, R.sub.2, and R.sub.12, respectively, a current flowing in an output terminal O.sub.1 has a value of (I.times.R.sub.1 /R.sub.2), and a current flowing in an output terminal O.sub.2 has a value of (I.times.R.sub.1 /R.sub.12). The two output current values disadvantageously vary at about several %.